In modern light-emitting devices, gallium nitride-based materials are very important wide-bandgap semiconductor materials, which can be applied to green, and blue to ultraviolet light-emitting devices. However, the formation of bulk gallium nitride is hitherto a technical bottleneck difficult to breakthrough. By using sapphire or silicon carbide (SiC) as a substrate, a gallium nitride layer can grow epitaxially onto the substrate. Nevertheless, a substantially high dislocation density exists in the gallium nitride layer fabricated using such a process, resulting in reduction in luminance efficiency and electron mobility, and thereby a light-emitting diode with relatively higher luminance efficiency cannot be attained.
In view of the considerations, U.S. Pat. No. 5,290,393 discloses forming a low-temperature gallium nitride buffer layer at temperatures between 200° C. to 900° C. and with a thickness of between 0.001 μm to 0.5 μm on a sapphire substrate, and then forming a high-temperature gallium nitride layer at temperatures between 900° C. to 1150° C. Although the method can increase crystallizability of the gallium nitride-based materials, the dislocation density of the whole gallium nitride layer is as high as 109 to 1010 cm−2. Another US patent with U.S. Pat. No. 6,252,261 discloses reducing dislocation density in the gallium nitride layer during subsequent epitaxial growth by using an epitaxial lateral overgrowth (ELOG) method. Although the method can reduce dislocation density effectively, an approximate thickness of 10 μm is required to reduce the dislocation density below 1×108 cm−2. In addition, production cost is relatively high because of mask fabrication in the processes and complex control mechanisms in selective growth. Furthermore, U.S. Pat. No. 6,475,882 discloses a lateral epitaxial technology using silicon nitride (SiN) micro-masks. However, the drawback of the technology disclosed in the patent is that the uniformity and the density of the SiN micro-masks are difficult to control, which will result in difficulty in controlling production yields. Moreover, U.S. Pat. No. 6,700,179 discloses a surface treatment technology using silicon as an anti-surfactant, which can attain better crystallization of subsequent epitaxial layers by controlling the reaction time of the reaction precursor SiH4. However, the drawback thereof is still difficulty in controlling uniformity as well as density. Bad uniformity tends to result in non-uniformity of stress distribution in the epitaxial layers, which in turn will result in difficulty in controlling production yields.
Consequently, in accordance with the problems described above, the present invention provides a structure and a manufacturing method of epitaxial layers of gallium nitride-based compound semiconductors.